Central neurons form and maintain specific connections, which are critical for the proper functioning of the nervous system. In Aplysia many central neurons are identified functionally because of their specific connections with central or peripheral targets. Aplysia sensory neurons in cell culture form specific synapses (with motor neuron L7 but not L11) even when they interact with both targets simultaneously. How do neurons establish specific synapses when interacting simultaneously with appropriate and inappropriate partners? Target-induced local secretion of a neuropeptide, sensorin, from sensory neurons and its autocrine signaling regulate presynaptic axonal growth associatedwith the formation and maintenance of specific sensory neuron synapses. Sensorin and its signaling pathway also regulate the distribution and expression of its own mRNA and protein and that of other mRNAs that encode proteins critical for synaptic function. Both sensorin mRNA and mRNAs for some of the proteins critical for synaptic function are also transported to terminals, and target-dependent regulation of mRNA localization and local translation contribute to the formation of sensory neuron synapses. Thus a retrograde signal from a specific postsynaptic target neuron regulates the expression, secretion and signaling of a presynaptic neuropeptide, which in turn regulates a number of cellular processes required for synapse-associated growth and the formation of specific synapses. Using cellular methods (electrophysiology, pharmacology, immunostaining and light microscopy) and molecular techniques (RT-PCR, in situ hybridization, anti-sense knock down methods, and expression of GFP-tagged constructs), we plan to use a model cell culture system containing identified neurons of the marine mollusk Aplysia to test the above hypothesis by examining the following aims: 1) To determine how the appropriate target regulates the expression and secretion of the neuropeptide. 2) To determine the signaling pathways that contribute to the formation of specific synapses. 3) To determine how the neuropeptide regulates cellular processes in both presynaptic neuron and target that are associated with the formation of synapses. The results will provide fundamental understanding into the normal cellular processes that allow the proper development of neural circuits that regulate behavior. Failure to establish specific synaptic connections during development may contribute to several disorders of nervous system function expressed either during early life or only after maturation.